Last Tuesday, Bill Nye "The Science Guy" faced off against Ken Ham of "Answers in Genesis" in a highly publicized debate about the following question:

Is creation a viable model of origins in today's modern, scientific era?

In other words, it was evolution versus creation, a debate that's surfaced in various forms since Darwin. Last week's manifestation, however, was a peculiar blend of old and new, with both parties emphasizing the importance of science for contemporary technological innovation, but Ham representing a rather old-school form of creationism — one according to which God created the world in six literal days some 6,000 years ago. (Even televangelist Pat Robertson balked, calling Ham's view "nonsense.")

When it comes to the evidence for evolution, I don't have much to say. To quote Chris Mooney, "the case for evolution is a slam dunk." But the debate did reveal some surprising assumptions about the nature of science — on Ham's part — that have received less attention in debate postmortems, and that are worth some careful scrutiny.

Ham began the debate by emphasizing the importance of defining one's terms, including the term "science." In particular, he made two strong claims about science and science education, captured on the presentation slide below.

The first claim is that we should recognize an important distinction between "observational science," which concerns the here and now, and "historical science," which concerns origins, or the past. The second claim is that science is arbitrarily defined in a way that outlaws the supernatural.

To get a sense of the status of these ideas in contemporary science and philosophy, I turned to my friend Michael Weisberg, a philosopher of science and associate professor of philosophy at the University of Pennsylvania.

Weisberg pointed out that while you can differentiate historical from nonhistorical science, the distinction might not be a deep one:

"Philosophers of science certainly recognize differences between historical sciences and what Ham is calling 'observational sciences' (we often use terms like 'nomothetic' to describe sciences like chemistry and physics). The main difference is simply that historical sciences like geology and evolutionary biology try to reconstruct the past, while other sciences like chemistry aren't especially concerned with any particular time period. Whenever one tries to reconstruct an event that has happened in the past, the direct evidence base will be limited. By definition, historical events have already happened and we won't be able to go out and have a look at them.

"But very few studies, even of historical phenomena, are strictly limited to studying the historical record. For example, geologists re-create historical conditions in their laboratories to study the effects of these conditions, while evolutionary biologists might use genomic data to help reconstruct phylogenetic trees. So while there are certainly differences between these sciences, they are not fundamental."

For Ham, the differences run much deeper; he wants to treat the conclusions of observational science as fundamentally different from those of historical science. In particular, he's willing to accept the former but wants to question the latter. As he said multiple times throughout the debate: We weren't there to see what happened, so we could be wrong.

But as Weisberg points out, historical inferences can be very reliable:

"There are many things that we can't see for ourselves, but about which we can make reliable inferences. Scientific methods help us ensure the reliability of these inferences, often by ruling out other possible explanations (confounding factors) and by bringing multiple, independent lines of evidence forward. This can be quite challenging for historical sciences. Darwin, ever aware of this challenge, brought studies of morphology, physiology, paleontology, and biogeography together to form the basis of his evolutionary theories. Modern evolutionists can add genetics and development to the mix."

On the flip side, observational science can't offer guarantees, either. All scientific claims rest on defeasible assumptions. When we conclude that the Earth is round because it looks that way in photographs taken from space (one of Ham's examples, offered to reject the parallels between a historically young Earth and an observationally flat Earth), we're engaged in observational science, but it's science that rests on assumptions about optics and how our devices for measurement work, among other things. These happen to be assumptions that have held up very well over time and are supported by converging sources of evidence ... just like evolution.

In fact, all scientific hypotheses — whether observational or historical — are evaluated by their fit with what we observe. Scientists test out the implications of some hypothesis, and it's accepted, revised, or rejected accordingly. That's why Nye kept challenging Ham to produce predictions of his model of origins, and Ham claimed to do so. Toward the end of the debate, he summarized as follows:

"I actually went through some predictions, and listed others, and there's a lot more that you can look at and you can go and test it for yourself. If this book [the Bible] really is true, it is so specific, it should explain the world, it should make sense of what we see. The flood? Yeah, we have fossils all over the world. The Tower of Babel? Yeah, different people groups, different languages, they have flood legends very similar to the Bible, creation legends similar to the Bible, there's so much you can look at ..."

So — sometimes — Ham seemed to endorse the idea that his model of creation makes predictions about what we should find in the natural world, and that what we observe (the findings of observational science) are relevant to the truth of that model (see presentation slide below). More generally, he appeared to endorse the idea that observational science constrains claims in historical science, where those claims can be supernatural.

But this seems to be at odds with the idea that all bets are off when it comes to historical science, and also with his second claim about how science is defined: that it arbitrarily rules out the supernatural. If some claim makes observational predictions, then it's susceptible to scientific testing — that's arguably the hallmark of a scientific hypothesis.

So Ham can't have it both ways: on the one hand claiming that empirical evidence supports a historical claim of (supernatural) creation, and on the other that creation has been arbitrarily and a priori ruled out by textbook definitions of science.

This tension may have arisen from a peculiar feature of the debate. Nye and Ham spent much of their time contesting the evidence for evolution versus creation: the nature of the fossil record, the (im)plausibility of housing 7,000 kinds of animals on a boat built by Noah and his family, and so on. But their disagreement was much more fundamental, and concerned the very bases for belief, including the role of evidence versus revelation.

These issues made occasional appearances, but were never truly engaged. Ham seemed to be debating Nye with ground rules that he didn't actually accept: that observable evidence should bear on one's beliefs about origins.

When asked what could possibly change his mind, Ham responded:

"I'm a Christian."

In the first half of the debate, Ham accused evolutionists of hijacking the word evolution using a "bait and switch." But the bigger bait and switch was the premise of the debate itself, which Ham ultimately rejected: that observable evidence and argument can and should change minds.

At least, that's my take from watching a streamed version of the debate after it occurred. But of course, I can't say for sure. It was, after all, in the past, and I wasn't there to see what happened.

You can keep up with more of what Tania Lombrozo is thinking on Twitter: @TaniaLombrozo